1. Field of the Invention
The present invention relates to a wrapping apparatus. More particularly, the present invention relates to a wrapping apparatus for winding a wrapping film about an article to be wrapped.
2. Description of the Related Art
In prior art, a wrapping apparatus for winding a wrapping film about an article to be packaged is known. The wrapping apparatus comprises a film dispenser arranged to revolve along a circular endless track at a constant velocity about the article to be wrapped. The film dispenser comprises a frame, supporting elements for supporting a film roll on the frame, and a pre-stretching device. The pre-stretching device comprises a first pre-stretching roller rotatably mounted on the frame with bearings at both ends to receive the film from the film roll, and a second pre-stretching roller rotatably mounted on the frame with bearings at both ends and disposed in a position parallel to and at a distance from the first pre-stretching roller. The pre-stretching rollers are coupled together via a regular transmission so that their circumferential velocities differ from each other, the pre-stretching of the film thus occurring within the film portion between the pre-stretching rollers as a result of their different circumferential velocities. The film dispenser further comprises a pendulum roller disposed after the second pre-stretching roller in the direction of film movement to receive the pre-stretched film from the second pre-stretching roller. The pendulum roller is spring-loaded with a spring acting against the drawing direction of the film web. The film dispenser further comprises a deflecting roller mounted by both ends with bearings on the frame, in a position parallel to the pre-stretching rollers and the pendulum roller, the film web coming from the pendulum roller being passed over the deflecting roller to the article to be wrapped.
Typically, the product to be wrapped is a pallet and its cross-section is of a rectangular form. Therefore, the feed rate (i.e. the drawing speed of the film in relation to the film dispenser) at which the film is passed from the film dispenser onto the article varies as the film dispenser is revolving at a constant speed about the article. As the film is stretched between the rollers, the film portion after the pre-stretching rollers is strained to a tension that is proportional to the speed difference between the rollers, to the drawing velocity, to the thickness and width of the film and to the internal elongation properties of the film. As a result of the quadrangular form of the object being wrapped, e.g. a pallet, the drawing velocity of the film varies continuously, producing variation in film tension.
To eliminate the variation of tension, prior-art apparatus use a sensor connected to the pendulum roller to detect the degree of film tension. The film tension detected by the sensor is used as a basis to control the speed of rotation of either the motor driving the film roll, as in European Patent No. EP 0 936 141 A1. the motor driving the posterior pre-stretching roller as seen in the direction of film motion, as in U.S. Pat. No. 5,123,230A, or the drive motor of some other roller used to draw the film from the film roll, as in Patent No. WO 93/24373. These arrangements are designed to eliminate the variation in film tension resulting from the angular shape of the article to be wrapped and to achieve a constant film tension.
The problem with prior-art apparatus is that they are complex and expensive as they need tension detectors and drive motors controlled on the basis of these to drive the rollers and/or the film roll.